atan2 equivalent in Cycles nodes?
EDIT July 2018

If you are using Blender 2.8 or a recent build of 2.79, the math node now has the atan2 operation since this commit by Lukas Stockner.





This is in no way the most efficient way of doing it, but just a reminder that knowing a bit of math, you can painstakingly craft functions using NodeGroups.


Things to know:


The Maximum Math node acts as an OR boolean operation.
The Minimum Math node acts as an AND boolean operation.
The Substract Math node acts as a NOT boolean operation with 1 as first input and your value as second.
The Mix Colors node acts as a switch with a boolean as first input, which returns second input if condition == False, or third input if condition == True.


http://en.wikipedia.org/wiki/Atan2

http://en.wikipedia.org/wiki/De_Morgan%27s_law